Physico-chemical characteristics of mucus glycoproteins and lipids of the human oral mucosal mucus coat in relation to caries susceptibility

Mucus coat was isolated from oral epithelial surfaces of caries-resistant and caries-susceptible subjects, analysed for content and composition of lipids and mucus glycoproteins, and evaluated for physico-chemical characteristics. The mucus coat from caries-resistant subjects had a protein content similar to that of the caries-susceptible group but a higher content of carbohydrate and a lower content of lipids and covalently bound fatty acid. The carbohydrate component was mainly mucus glycoprotein, which accounted for 28.4% of the dry weight of caries-resistant mucus and 25.3% of caries-susceptible mucus. By Chromatographic analysis on Bio-Gel A-50, both types of preparations had high (M_r ≈ 2000 kdalton) and low (M_r ≈ 300 kdalton) molecular-weight mucus glycoproteins. In the caries-susceptible mucus coat these two glycoproteins were in similar proportions, whereas the low molecular-weight glycoprotein predominated in caries-resistant mucus. In both preparations, the high molecular-weight glycoprotein was characterized by a high content of carbohydrates, associated lipids and covalently bound fatty acids, whereas the low molecular-weight glycoprotein was richer in protein and contained lesser amounts of associated and covalently bound lipids. Although the low molecular-weight glycoprotein showed only minor compositional differences with caries status, the high molecular-weight glycoprotein of the caries-resistant group had a 2.5 times lower content of covalently bound fatty acid, a 1.3 times lower content of associated lipids and contained 1.2 times more sulphate and sialic acid then that of the caries-susceptible group. The mucus coat of caries-resistant individuals differed from that of caries-susceptible subjects with respect to viscosity, permeability and hydrophobicity. The caries-resistant mucus coat had 1.4 times lower viscosity, a 1.3-fold reduction in glucose diffusion retardation capacity and a lower binding affinity for a fluorescent hydrophobic probe. Similar differences in physical characteristics in relation to caries status were also displayed by the high molecular-weight mucus glycoproteins, but there were only minor differences between the low molecular-weight mucus glycoproteins of the two groups. Thus the oral mucosal mucus coat of caries-resistant individuals differ from that of caries-susceptible subjects in physico-chemical characteristics; these differences reflect the distribution of different mucus glycoprotein molecular forms and their content of associated and covalently bound lipids.